ADR-0017 implementation: per-cell type-change with override flags

Replaces the placeholder "trust STRICT" body of do_change_column_type
with the per-cell transformer matrix from ADR-0017. Adds:

- src/type_change.rs: CellOutcome { Clean / Lossy / Incompatible }
  + transform_cell + static_refusal covering every matrix pair
  from §3 (54 unit tests).
- --force-conversion and --dont-convert flags on `change column`
  (mutually exclusive at parse time per §5).
- Refined PK rule (§4.1): refused only when the column has an
  inbound FK and fk_target_type would change. Outbound-FK columns
  still refused outright (§4.2). PK / shortid uniqueness checked
  post-transformation (§4.3).
- Bordered diagnostic tables (lossy / incompatible / collision)
  via the pretty-table renderer (§7) — uses ADR-0016's primitives.
- [client-side] success note (§6) when any cell was rewritten.
- Friendly wrapper for engine-level errors under --dont-convert
  so no engine vocabulary leaks (ADR-0002 user-facing posture).

ADR-0017 §3 + §7 amended in place (with user sign-off): serial->int
added explicitly to the always-clean matrix, and diagnostic rows
identify themselves by PK value(s) rather than positional indices
(SQLite returns rows unordered without ORDER BY, so positional
"row 5" is unaddressable).

Tests: 449 -> 517 (+68). Clippy clean with nursery lints.

src/event.rs

@@ -7,7 +7,10 @@
 
 use crossterm::event::KeyEvent;
 
-use crate::db::{DataResult, DeleteResult, InsertResult, TableDescription, UpdateResult};
+use crate::db::{
+    ChangeColumnTypeResult, DataResult, DeleteResult, InsertResult, TableDescription,
+    UpdateResult,
+};
 use crate::dsl::Command;
 
 #[derive(Debug, Clone)]
@@ -39,6 +42,13 @@ pub enum AppEvent {
         command: Command,
         result: DeleteResult,
     },
+    /// A `change column …` succeeded. `result` carries both the
+    /// post-rebuild description (for the auto-show) and the
+    /// optional `[client-side]` note (ADR-0017 §6).
+    DslChangeColumnSucceeded {
+        command: Command,
+        result: ChangeColumnTypeResult,
+    },
     /// A DSL command failed. `error` is already a friendly
     /// message produced via `DbError::friendly_message`.
     DslFailed {